Fluid filters consisting of a filtering element interposed in a fluid path exist in many forms. The fluid path typically has a reservoir or chamber to receive fluid on the inlet side of a filter element and a housing to hold and support the filter element. On the outlet or discharge side of the filter element the fluid path commonly includes another chamber with an outlet port to receive the clean filtered fluid and convey the fluid away. The filtration process requires either applying positive-pressure from a pump or a gravity-feed on a fluid stream to push it through the inlet chamber and the filter element, or negative-pressure applied at the outlet side to pull fluid through the filter element.
Filters are commonly used to remove particulate matter from fluid streams in order to prepare clarified fluid. The filter element is typically enclosed in a cartridge or housing to protect it and the fluid content from external exposure and contaminates. Enclosed or encapsulated cartridge-type filters of this type with inlet and outlet chambers and respective inlet and outlet ports for the fluid path are often referred to as in-line fluid filters.
There are common problems associated with fluid filtration using enclosed filter cartridges or capsules, especially those having small internal volume, e.g., <5 mL. For example, to process aqueous solutions the typical filter elements are hydrophilic porous membranes. Such membranes once wetted do not typically pass air under normal conditions of use. Difficulties are encountered if air enters the inlet chamber of an enclosed filtration apparatus after the initial introduction of fluid. Trapped air will impede and can completely prevent flow. Air in contact with the filter membrane occludes fluid flow. An accumulation of air covering the filter element creates an “air-lock” that completely blocks fluid flow, regardless of whether the filtration is based on a negative or positive-pressure fluid flow approach. Vacuum at the outlet side cannot pull the blocking air through the filter element to allow fluid to enter the inlet chamber, which is needed to resume flow. Nor can fluid necessarily be pushed by positive pressure into an air-locked inlet chamber because of the air blocking the fluid path.
Several approaches have been designed to alleviate the problem of air blockage and locking in filtration devices with enclosed inlet chambers. One is to incorporate a passive vent into the inlet chamber so that there is an air release or escape path. For aqueous filtration, such devices may cover the vent hole with a porous hydrophobic membrane, e.g., PTFE, which prevents fluid passage and leakage from the device, but allows air to exit via the vent.
The inlet-chamber passive vent generally only works, however, if the filtration system is based on fluid positive pressure and not a negative-pressure approach. It requires that fluid be pushed in via the inlet port and relies on that flow to dispel the air from the inlet chamber such that fluid contacts the filter element and hence passes through it. In contrast, regardless of venting of the inlet chamber, vacuum applied to the outlet side of a wetted membrane is ineffectual for ridding an air-locked inlet chamber of air and allowing fluid flow. A vacuum at the outlet will not pull air through the wetted membrane and will not create a negative pressure in the inlet chamber. Moreover, it is impractical because the passive vent on the inlet chamber constitutes a pathway for air to leak into or be pulled into the inlet chamber as fluid is drawn through, thus promoting air-lock conditions.
Another approach for voiding air from an inlet chamber and circumventing air locks has been the design of special vented filter elements. These incorporate regions that are hydrophilic for fluid filtration, and also have hydrophobic regions specifically for venting air through the filter. These devices avoid problems of complete locking and can be used with either positive or negative-pressure based filtration system. However, their shortcomings are that air passage through the membrane may be undesirable, inclusion of the venting regions reduces fluid flow efficiency and may be only partially effective at voiding trapped air, and the special membrane is higher cost.
In summary, problems are encountered with current devices and practices that impact filter performance and impede or completely block fluid flow in filtration devices with enclosed inlet chambers. There are partial solutions using vented cartridges and other deficiencies with vented-filters, which are the alternative for negative-pressure systems.
The present invention overcomes these insufficiencies in a practical manner enabling enclosed filtration devices to be used with both positive pressure, and more attractively, negative-pressure filtration systems in applications that heretofore were untenable. It does so by providing simple and practical means for actively applying negative-pressure to the inlet chamber to aspirate air and/or fluid while either negative or positive pressure is independently employed to move fluid through the filter element.